The Global Navigation Satellite System (GNSS) generically includes the Global Positioning System (GPS) operated by the United States, the Global Orbiting Navigation Satellite System (GLONASS) operated by the Russian Federation, and various other systems intended to increase the number of satellite- or ground-based ranging sources, or otherwise to augment the performance of user equipment operating with some or all of these systems.
The GPS and GLONASS, and certain other augmentations to GPS and GLONASS, operate in the portion of the radio spectrum above 1 GHz. GNSS ranging signals, received by typical user equipment located near the Earth surface, have power levels on the order of 10−16 Watt. A practical concern for users of these systems is signal attenuation by natural objects such as trees, and manmade objects such as buildings. When signals are strongly attenuated by intervening objects, they are more difficult to track and data modulated on these signals are more difficult to read without error. Strong attenuation can prevent successful operation of the user receiving equipment.
A conventional GNSS receiver performs three basic functions:                a) it estimates the code phase (pseudorange) for each of several ranging signals at one or several instants of time;        b) it demodulates navigation data transmitted by the ranging sources, which provide information as to the locations of said ranging sources at the instants of time at which the pseudorange measurements were made; and        c) it computes the position of the receiving antenna by a process of triangulation, using the known locations of the ranging sources and the estimated pseudoranges to those sources.        
A typical GNSS receiver tracks each of the several ranging signals separately, using for example an early/late gate Delay Lock Loop (DLL) implementation to estimate signal arrival time and for example a Costas-type Phase Lock Loop (PLL) to estimate signal carrier phase. The DLL typically despreads the ranging signal for the PLL, and the PLL provides a carrier reference for data demodulation. The DLL can lose lock at low signal-to-noise ratios; a typical performance level for desirable operation is in the range of 30 dBHz. The demodulator requires a reference oscillator “locked” to the incoming ranging signal. The PLL can also lose lock at low signal-to-noise ratios, and the demodulator can experience bit errors at low signal-to-noise ratios even when the phase-lock loop is stable. A typical performance level for desirable operation, for the PLL and also the demodulator, is in the range of 30 dBHz.
When the user receiver is shadowed from the transmitting ranging sources, for example by trees or manmade structures, signal level is reduced, tracking loops become unstable, and receiver performance can degrade. In many user environments, ranging signals also arrive by multiple reflected paths as well as a direct path. The signals arriving by reflected paths can “fool” the tracking loops and degrade the performance of the receiver.
One solution previously developed to mitigate low signal strength and tracking loop instability is known as vector tracking or Integrated Demodulator Navigation. That solution is taught in U.S. Pat. No. 5,343,209 to Sennott et al. In this scheme “[T]he coupled-tracking navigation receiver periodically measures carrier phase, carrier frequency, modulation phase, and carrier amplitude for all of the signals arriving at the receiving ports of the receiver and periodically estimates the present values of carrier phase, carrier frequency, and modulation phase for all of the received signals, the estimating process utilizing for each parameter estimate the parameter measurements for a plurality of the received signals properly combined in a statistically appropriate manner by taking into account the relative geometry of the line-of-sight paths, receiver clock time dynamics, and dynamics and motion constraints of the receiver platform, thereby obtaining better performance under poor signal reception conditions and more accurate estimates of carrier phase, carrier frequency, and modulation phase for each of the received signals than independent measurements alone could provide. It follows that these more accurate estimates of the basic signal parameters lead to more accurate estimates of platform position and attitude and the rates of change of these quantities.”
Another solution, recently developed to mitigate low signal strength, is known as SnapTrack™. The Snaptrack concept is disclosed in U.S. Pat. Nos. 6,064,336, 6,061,018, 6,052,081, 6,016,119, 6,002,363, 5,999,124, 5,945,944, 5,884,214, and 5,874,914. It is also described in a technical article by Moeglein and Krasner, “An Introduction to SnapTrack™ Server-Aided GPS Technology.” Snaptrack achieves high sensitivity, low time-to-first-fix and low power dissipation in comparison to a conventional GPS receiver by relying on aiding information provided by an external location server. The aiding information may consist of a stable carrier reference signal, satellite ephemeris information (i.e., so that the user receiver is not required to demodulate this information), and observation windows for the various ranging signals expected to be observable by the user receiver. Pseudorange information (essentially, relative arrival time estimates extracted by a ranging code cross-correlation process at the user receiver) is typically passed back to a remote location server for position estimation. However, if the navigation data associated with the ranging signals are made available to the mobile receiver, the position estimation function can be performed locally.
A characteristic of the Snaptrack concept is that the remote receiver does not “track” the ranging signals in the classical sense. Instead, the remote receiver performs a cross-correlation against a known ranging code within an observation window specified by the remote location server. The peak or peaks of the cross-correlation function within the observation window, for each ranging signal observed, is (are) used for position estimation either locally or at the remote location server. Since tracking in the classical sense is not performed, the Snaptrack receiver avoids the instability of a tracking circuit. This allows the overall position determination system to operate at a signal level that is substantially weaker than the level required by a conventional GPS receiver. However, the location server may need to provide new information regarding the desired observation windows, for each ranging signal, for each new position fix. If the Snaptrack receiver attempts to adjust the observation windows based on its current observations, it is essentially tracking the ranging signals and classical tracking instability may ensue.
One method known in the prior art, to accurately estimate parameters such as signal arrival time, is MUltiple SIgnal Characterization (MUSIC) described by Schmidt, Ralph Otto, 1981, “A Signal Subspace Approach to Multiple Emitter Location and Spectral Estimation” (Stanford University). An Iteratively CONvergent (ICON) improvement of MUSIC and similar algorithms, also known in the prior art, has been previously disclosed by the inventor (Heppe, Stephen B., 1989, “Iteratively Convergent Methods of Signal Characterization Based on Eigenspace Analysis” (The George Washington University)).